1. Field of the Invention
The present invention relates to a transistor and a display device including the transistor. More particularly, the present invention relates to a transistor having an organic layer as a semiconductor layer including a conductive polymer and also relates to a display device including such a transistor. As used herein, the term “conductive polymer” refers to a novel polymer that behaves as a conductor or a semiconductor.
2. Description of the Related Art
Recently, an active-matrix liquid crystal display device (LCD) that uses active components such as thin-film transistors (TFTs) as its switching elements has frequently been used as a monitor for notebook computers, desktop computers, workstations and so on, and in a liquid crystal television. This is because the active-matrix LCD can display an image having high quality, which is at least comparable to, or even better than, an image displayed on a CRT, while consuming lower power and occupying a smaller space than the CRT. Despite these advantages, the active-matrix LCD is still much more expensive than the CRT. Accordingly, in order for this type of display device to become even more popular, its price must be reduced.
Meanwhile, an active-matrix organic EL (electro-luminescence) display device has also been vigorously researched and developed recently. In the active-matrix organic EL display device, the light-emitting element thereof (i.e., an active-matrix organic light-emitting diode (OLED)) is driven by an active component. Thus, the active component for such an organic EL device also needs to be manufactured at a lower cost.
One of the proposed methods of reducing the manufacturing cost of the active-matrix LCD or organic EL device is the use of an organic field effect transistor (FET) or an organic thin-film transistor (TFT) as the active component. The organic FET or organic TFT includes an organic semiconductor thin film, which can be easily formed by a currently available thin-film deposition technique.
However, a plasma-enhanced chemical vapor deposition (CVD) system, which is often used today to form an insulating layer or a semiconductor layer for an amorphous silicon (a-Si) TFT or a polysilicon (p-Si) TFT, and a sputtering system for use to form electrodes for such a TFT are both expensive. Also, in a CVD process, the insulating or semiconductor layer needs to be deposited at a temperature as high as about 230° C. to about 350° C. Furthermore, the CVD or sputtering system normally requires frequent maintenance such as cleaning, thus resulting in a low throughput. In contrast, a coater or an inkjet printer for use in forming an organic FET, for example, is much less expensive, and requires much simpler maintenance, than the CVD or sputtering system. Also, the organic semiconductor thin film can be deposited at a far lower temperature than the conventional insulating or semiconductor layer formed by the CVD process. Accordingly, when an organic FET is used for a liquid crystal display device or an organic EL display device, significant cost reduction is expected.
An organic TFT normally includes a transparent substrate (e.g., a glass substrate), a gate electrode, a gate insulating film, a source electrode, a drain electrode and an organic semiconductor film. In the organic TFT, by changing the voltage to be applied to the gate electrode, the quantity of charge existing in the interface between the gate insulating film and the organic semiconductor film is adjusted and thereby the amount of current flowing from the source electrode into the drain electrode by way of the organic semiconductor film can be changed. In this manner, the organic TFT can be selectively turned ON or OFF, and is also referred to as an “organic FET”.
For example, Yen-Yi Lin, David J. Gundlach, Shelby F. Nelson and Thomas N. Jackson disclosed in IEEE Transactions on Electronic Devices, Vol. 44, No. 8, p. 1325 (1997) that an organic TFT was formed using pentacene.
However, an organic semiconductor film formed of pentacene must be deposited by an evaporation process. Accordingly, to improve the characteristics of such a film, the film needs to be crystallized sufficiently. On the other hand, according to another proposed technique, a pentacene derivative is used as an alternative organic film material to make the resultant organic film soluble and patternable more easily. In order to deposit a thin film of a low-molecular-weight organic semiconductor material such as pentacene, an evaporation process must be performed. Thus, the organic TFT cannot be manufactured efficiently. Also, since such a material normally cannot be crystallized sufficiently, desired characteristics are not always achieved.
Furthermore, Japanese Laid-Open Publication No. 63-76378 discloses a method of fabricating an organic TFT by using polythiophene or a derivative thereof for its organic semiconductor film.
An organic semiconductor thin film made of polythiophene, a polythiophene derivative or a thiophene oligomer can be formed easily by an electrochemical polymerization process or a solution coating technique. That is to say, such an organic semiconductor is an easily patternable material. However, such a material is still under research and development, and the properties thereof are not yet completely satisfactory.
Generally speaking, many of the conventional conductive polymers are rigid, insoluble and infusible. Accordingly, to introduce or increase the fusibility or solubility, a polymer derivative or oligomer with additional side chains is sometimes used (see Japanese Laid-Open Publications Nos. 4-133351, 63-76378 and 5-110069, for example). However, when the side chains are introduced, the polymer chain will increase its flexibility excessively. Then, the glass transition point will be included within the operating temperature range. As a result, thermochromism will be created due to micro-Brownian motion, π electrons will increase their conjugate length, and the stability of characteristics against temperatures will decrease.
On the other hand, when an oligomer is used, the reliability of the conductive polymer adversely decreases. In addition, a material with the additional oligomer cannot ensure a sufficient mobility. Accordingly, to increase the mobility, either the degree of polymerization or the alignment property of the conductive organic compound should be increased. The alignment property of the conductive organic compound may be improved by additionally providing an alignment film as disclosed in Japanese Laid-Open Publication No. 7-206599, for example.
Furthermore, π conjugated polymers are easily affected and deteriorated by oxygen or water in the air.
As described above, the conventional organic TFTs exhibit insufficient electrical characteristics, lack in chemical stability, and have a short lifetime.
Recently, a polymer complex, which is obtained by complexing together either multiple dissimilar polymers or a high-molecular-weight compound and a low-molecular-weight compound through non-covalent interactions, has attracted much attention as a novel functional material in certain specific fields not related to transistor technology or display technology. For example, a polymer inclusion complex, including a conductive polymer as a guest molecule and cyclodextrin (CD, which is an insulating cyclic molecule) or a molecular nanotube, synthesized from cyclodextrin, as a host molecule, respectively, is expected to be applicable as a molecular conductor. See Abe et al., “Structure And Physical Properties Of Molecular Wire Consisting Of Polyaniline And Molecular Nanotube”, Polymer Preprints, Japan, Vol. 50, No. 12 2980 (2001) and Shimomura et al., “Molecular Coated Wire Consisting Of Conductive Polymer And Molecular Nanotube”, Polymer Preprints, Japan, Vol. 50, No. 13, 3265 (2001), for example. However, nobody has ever reported or suggested that an organic TFT or display device could or should be formed by using a semiconductor layer made of a polymer inclusion complex.